The invention relates generally to an electrophotographic printing machine and, more particularly, to a development system which includes a magnetic developer roll for transporting soft magnetic developer materials to a development zone; and a magnetic system for generating a magnetic field to reduce developer material bed height in the development zone. To overcome or minimize such problems, the soft magnetic developer materials of the present invention were arrived at after extensive research efforts, and which soft magnetic developer materials result in, for example, sufficient particle charge for transfer and maintain the mobility within the desired range of the particular imaging system employed.
Generally, an electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing the document being produced. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic latent image is formed on the a photoconductive member, the image is developed by bringing a developer material into proximal contact therewith. Typically, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted to the latent image from the carrier granules and form a powder image on the photoconductive member which is subsequently transferred to a copy sheet. Finally, the copy sheet is heated or otherwise processed to permanently affix the powder image thereto in the desired image-wise configuration.
In the prior art, both interactive and non-interactive development has been accomplished with magnetic brushes. In typical interactive embodiments, the magnetic brush is in the form of a rigid cylindrical sleeve which rotates around a fixed assembly of permanent magnets. In this type of development system, the cylindrical sleeve is usually made of an electrically conductive, non-ferrous material such as aluminum or stainless steel, with its outer surface textured to control developer adhesion. The rotation of the sleeve transports magnetically adhered developer through the development zone where there is direct contact between the developer brush and the imaged surface, and charged toner particles are stripped from the passing magnetic brush filaments by the electrostatic fields of the image.
These systems employ magnetically hard ferromagnetic material, for example U.S. Pat. No. 4,546,060 discloses an electrographic, two-component dry developer composition comprising charged toner particles and oppositely charged, magnetic carrier particles, which (a) comprise a magnetic material exhibiting xe2x80x9chardxe2x80x9d magnetic properties, as characterized by a coercivity of at least 300 gauss and (b) exhibit an induced magnetic moment of at least 20 EMU/gm when in an applied field of 1000 gauss, is disclosed. Magnetically xe2x80x9chardxe2x80x9d carrier materials include strontium ferrite and barium ferrite, for example. These carrier materials tend to be electrically insulative as employed in electrophotographic development subsystems. The developer is employed in combination with a magnetic applicator comprising a rotatable magnetic core and an outer, nonmagnetizable shell to develop electrostatic images.
Non-interactive development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color or cross-contaminating the color toner supplies.
It has been observed in systems employing magnetically hard ferromagnetic material that the magnetic brush height formed by the developer mass in the magnetic fields on the sleeve surface in this type development system is periodic in thickness and statistically noisy as a result of complex carrier bead agglomeration and filament exchange mechanisms that occur during operation. As a result, substantial clearance must be provided in the development gap to avoid photoreceptor interactions through direct physical contact, so that the use of a closely spaced development electrode critical to high fidelity image development is precluded. The effective development electrode is essentially the development sleeve surface in the case of insulative development systems although for conductive magnetic brush systems the effective electrode spacing is significantly reduced.
It has also been found that in the fixed assembly of permanent magnets, the magnetic pole spacing thereof cannot be reduced to an arbitrarily small size because allowance for the thickness of the sleeve and a reasonable mechanical clearance between the sleeve and the rotating magnetic core sets a minimum working range for the magnetic multipole forces required to both hold and tumble the developer blanket on the sleeve. Since the internal pole geometry defining the spatial wavelength of the tumbling component also governs the magnitude of the holding forces for the developer blanket at any given range, there is only one degree of design freedom available to satisfy the opposing system requirements of short spatial wavelength and strong holding force. Reducing the developer blanket mass by supply starvation has been found to result in a sparse brush structure without substantially reducing the brush filament lengths or improving the uneven length distribution.
The above problems with controlling developer bed height are exacerbated when magnetically soft carrier material is employed. Such as disclosed in U.S. Pat. No. 6,143,456; U.S. Pat. No. 4,937,166; U.S. Pat. No. 4,233,387; U.S. Pat. No. 5,505,760; and U.S. Pat. No. 4,345,014 which are hereby incorporated by reference. U.S. Pat. No. 4,345,014 discloses a magnetic brush development apparatus which utilizes a two-component developer of the type described. The magnetic applicator is of the type in which the multiple pole magnetic core rotates to effect movement of the developer to a development zone. The magnetic carrier disclosed in this patent is of the conventional variety in that it comprises relatively xe2x80x9csoftxe2x80x9d magnetic material (e.g., magnetite, pure iron, ferrite or a form of Fe3O4) having a magnetic coercivity, Hc, of about 100 gauss or less. Such soft magnetic materials have been preferred heretofore because they inherently exhibit a low magnetic remanance, BR, (e.g., less than about 5 EMU/gm) and a high induced magnetic moment in the field applied by the brush core.
It is desirable to use magnetically soft carrier material because having a low magnetic remanence, soft magnetic carrier particles retain only a small amount of the magnetic moment induced by a magnetic field after being removed from such field; thus, they easily intermix and replenish with toner particles after being used for development. Additionally, conductive carrier material options are significantly broadened for the xe2x80x9csoftxe2x80x9d magnetic carriers. Also having a relatively high magnetic moment when attracted by the brush core, such materials are readily transported by the rotating brush and are prevented from being picked up by the photoconductive member during development.
Insulating magnetic brush (IMB) development using soft magnetic carriers having an insulating coating suffers from the shortcoming that it produces only relatively low developed mass/unit areas (DMA""s). This is due to the buildup of countercharges on the carrier beads as charged toner is developed from them onto the xerographic latent image. Development decreases with time for a given carrier bead until the point at which the attractive field due to the countercharges balances the attractive development field due to the photoreceptor. At this point, the contribution of a particular carrier bead to the development of a latent image ceases.
This problem was partially overcome by the invention of MAZE (magnetically agitated zone) development by Knapp, et. al. In MAZE development carrier bead chains (or bristles) are caused to tumble by changing the direction of the magnets inside the developer roll. As the chains tumble, they expose new carrier beads to the latent image, thereby partially overcoming the low latent images given by IMB development systems. However, even in MAZE development, the amount of dma developed onto the photoreceptor is still only 30-50% of that dictated by the field-collapse (i.e., the CMB) limit. Thus, there is still considerable room for improvement in the dma""s produced by (insulating) MAZE development systems.
Conductive magnetic brush (CMB) development systems allow the neutralization of the countercharges on carrier beads via conduction through the carrier bead chains. Thus, CMB development systems don""t suffer from the low dma problems of IMB systems. Indeed, applicants have found that CMB systems can develop to the field collapse limiting dma""s if sufficient numbers of development rolls are used. This may require 5-6, or more, rolls however. This is because of depletion of available toner from the developer bed near the ends of the carrier bead chain where in contact the photoreceptor.
A solution to this problem would be to use conductive carrier in MAZE. In this case we would expect that by tumbling the carrier bead chains so that 5 or more different carrier bead chain transitions within the development zone region in close proximity to the P/R we would be able to achieve the same dma""s as would be obtained from 5 or more development rolls. In effect this would overcome the supply limitations of a single development roll and reduce the effects of electrostatic field collapse. It would also enable higher process speed since toner replenishment at the surface of the mag brush roll would be improved.
While this approach sounds obvious and promising, conductive MAZE and TurboMAZE experiments have not proven effective. Two problems have been found with the conductive carriers, that are also magnetically xe2x80x9csoftxe2x80x9d, in MAZE development: First, the carrier on the developer roll tends to form bands, so that some areas of the roll have too thick a bed of developer (i.e., carrier beads plus toner) while other areas have none.
Secondly, the carrier on the developer roll tends to solidify and form an almost solid mass, precluding rotation of the developer roll, carrier bead chain rotation, and also replenishment of carrier on the development roll from carrier in the sump.
The following disclosures may be relevant to the present invention:
U.S. Pat. No. 5,890,041 discloses a development system for developing an image with developer material including a housing containing developer material; and a magnetic roll for transporting the developer material from the housing to the image, the magnetic roll including an magnetic core and a cylindrical sleeve enclosing and rotating about the magnetic core, the sleeve having a thickness between 0.001 to 0.006 inches.
U.S. Pat. No. 5,946,534 discloses a method for creating a densely packed, stable [non-bead chain forming] monolayer developer bed in the TurboMaze configuration. This configuration is achieved by designing carrier beads such that the bead to bead interaction is significantly less than the bead to magnetic substrate interaction by encapsulating a hard ferrite carrier bead in a nonmagnetic shell.
The present invention obviates the problems noted above by utilizing a development system, an electrographic, two-component dry developer composition comprising charged toner particles and oppositely charged carrier particles which (a) comprise: a soft magnetic material exhibiting a coercivity of less than 300 gauss when magnetically saturated and (b) exhibit an induced magnetic moment of less than 20 EMU/gm of carrier when in an applied field of 1000 gauss; and a hard magnetic material exhibiting a coercivity of at least 300 gauss when magnetically saturated and (b) exhibit an induced magnetic moment of at least 20 EMU/gm of carrier when in an applied field of 1000 gauss.